Buck began his career as a sitcom writer. He joined the staff of Six Feet Under as a writer and supervising producer in 2002 for the show's second season. He wrote the second season episode "It's the Most Wonderful Time of the Year". He remained a supervising producer for the third season in 2003 and wrote two further episodes "You Never Know" and "Everyone Leaves". He was promoted to co-executive producer for the fourth season in 2004. He wrote two more episodes "That's My Dog" and "Bomb Shelter". He was nominated for both an individual WGA award, as well as WGA awards for Best Writing Staff. He received a Peabody Award for his work on Six Feet Under. He remained a co-executive producer for the fifth and final season in 2005 and contributed two more episodes – "Dancing for Me" and "Singing For Our Lives". He contributed seven episodes to the series in total.

Buck worked as a co-executive producer on the second season of HBO's Rome in 2007. He wrote two episodes for the series ("These Being the Words of Marcus Tullius Cicero" and "Death Mask") before it was canceled.

He moved on to work as a co-executive producer and writer on the second season of Showtime's Dexter later in 2007, remaining as a co-executive producer for the third season in 2008, and an executive producer for the fourth season in 2009. In 2008, 2009 and 2010, Dexter was nominated for a Writers Guild of America Award for Best Dramatic Series (with the writing team including Buck credited on the nomination) but lost to The Wire and Mad Men.[1][2][3][4][5] For season six of Dexter, after the previous show runners left the series, Buck was promoted to showrunner position. His three seasons as showrunner were not well received, with the final season in particular receiving largely negative reviews.[6][7]

In December 2015, Buck was hired to serve as showrunner and executive producer for the Marvel and Netflix television series Iron Fist.[8] Following the series' debut online via streaming, reviews for Iron Fist were predominantly negative with critics calling it "easily the worst of the Netflix Marvel shows."[9] Despite this, Netflix reported that Iron Fist was their most-binged drama premiere.[10] In December 2016, it was announced that he would return to the Marvel Cinematic Universe as the showrunner and executive producer for the Marvel, IMAX Corporation, and ABC television series Inhumans.[11] In July 2017, it was announced that Buck was removed from his position as showrunner and executive producer on the TV series Iron Fist. Inhumans premiered in September 2017 to similarly negative reception, and was canceled after one season.